1. Field of the Invention
The present invention relates to an organic EL display panel.
2. Description of the Related Art
As display panels capable of high-quality display and replacing liquid crystal display panels, organic EL display panels using organic EL (Electro-Luminescent) elements have received a great deal of attention.
The organic EL display panels are classified into passive display panels having a passive matrix structure in which organic EL elements emitting light are located at intersections of anodes and cathodes, and active display panels which control a direct current to be supplied to organic EL elements by using thin-film transistors (TFTs).
A conventional active organic EL display panel will be described below. In this organic EL display panel, unit pixels each including an organic EL element are arranged in a matrix form in a display area where an image is displayed. FIG. 10A is a plan view of a unit pixel 200 in this organic EL display panel. FIG. 10B is an equivalent circuit of the unit pixel 200.
The unit pixel 200 includes an organic EL element 1, storage capacitor 2, EL driving TFT 3, and switching TFT 4. A power supply line 5 and a signal line 7 are arranged along the vertical direction in the display area. A scan line 6 is arranged along the horizontal direction. The power supply lines 5, signal lines 7, and scan lines 6 are arranged at predetermined intervals in the display area.
Referring to FIGS. 10A and 10B, the organic EL element 1 is connected to the power supply line 5 through the EL driving TFT 3. A power supply voltage is supplied from the power supply line 5 to the organic EL element 1 through the EL driving TFT 3. A signal voltage is supplied from the signal line 7 to the gate of the EL driving TFT 3 through the switching TFT 4. A control voltage is supplied from the scan line 6 to the gate of the switching TFT 4.
FIG. 11 is a sectional view taken along a line A-A′ in the organic EL element 1 shown in FIG. 10A. The organic EL element 1 includes a hole transporting layer 13, light-emitting layer 14, and electron transporting layer 15. The organic EL element 1 is arranged between a pixel electrode 12 and a common electrode 16. The pixel electrode 12 made of an ITO (Indium Tin Oxide) thin film is formed on a glass substrate 11 which extends over the display area. The hole transporting layer 13, light-emitting layer 14, electron transporting layer 15, and common electrode 16 made of a metal are formed on the pixel electrode 12 in this order. When a direct current is supplied to this structure, holes are emitted from the pixel electrode 12, and electrons are emitted from the common electrode 16. The holes and electrodes recombine in the light-emitting layer 14 and excite organic molecules in the light-emitting layer 14. Accordingly, light exits in the direction indicated by an arrow in FIG. 11.
Referring to FIG. 10A, the storage capacitor 2 includes an electrode 2a and an electrode 2b facing the electrode 2a. The electrode 2a is connected to the power supply line 5. The electrode 2b is connected to the drain of the switching TFT 4. When charges are stored between the electrode 2a and the electrode 2b, a current amount necessary for causing the organic EL element 1 to emit light is set.
In the organic EL display panel, no light is emitted from the region occupied by the storage capacitor 2, EL driving TFT 3, and switching TFT 4. If the area occupied by the storage capacitor 2, EL driving TFT 3, and switching TFT 4 in the unit pixel 200 is large, as shown in FIG. 10A, the ratio of the area of the organic EL element 1 which emits light becomes low. That is, the aperture ratio (the ratio of the area occupied by the organic EL elements 1 to the area of the display area) decreases. When the aperture ratio is low, the luminance per unit pixel 200 must be increased to maintain the luminance. This is a contributing factor to shorten the life of the organic EL element 1 and, accordingly, the life of the organic EL display panel.
Jpn. Pat. Appln. KOKAI Publication No. 2000-397475 (p. 6, FIG. 2) (patent reference 1) discloses an organic EL display panel, in which the thin-film transistor and storage capacitor are superposed on the light emission surface side of the display area to increase the aperture ratio and prolong the life.
According to the technique described in patent reference 1, the area occupied by the thin-film transistor is surely not a cause of a low aperture ratio. However, the storage capacitor having a larger area still suppresses the aperture ratio low. The effect for prolonging the life of the organic EL display panel is insufficient.
The present invention has been made to solve the problem, and has as its object to provide an organic EL display panel having a longer life.
(Patent Reference 1)
Jpn. Pat. Appln. KOKAI Publication No. 2000-397475 (p. 6, FIG. 2)